Microsoft Security Bulletin MS99-058 - Critical

Patch Available for "Virtual Directory Naming" Vulnerability

Published: December 21, 1999

Version: 1.0

Originally Posted: December 21, 1999

Summary

Microsoft has released a patch that eliminates a vulnerability in Microsoft® Internet Information Server and other products that run atop it. Under certain conditions, the vulnerability could cause a web server to send the source code of .ASP and other files to a visiting user.

If a file on one of the affected web server products resides in a virtual directory whose name contains a legal file extension, the normal server-side processing of the file can be bypassed. The vulnerability would manifest itself in different ways depending on the specific file type requested, the specific file extension in the virtual directory name, and the permissions that the requester has in the directory. In most cases, an error would result and the requested file would not be served. In the worse case, the source code of .ASP or other files could be sent to the browser.

This vulnerability would be most likely to occur due to administrator error, or if a product generated an affected virtual directory name by default. (Front Page Server Extensions is one such product). Recommended security practices militate against including sensitive information in .ASP and other files that require server-side processing, and if this recommendation is observed, there would be no sensitive information divulged even if this vulnerability occurred. In any event, an affected virtual directory could be identified during routine testing of the server.

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